1. Field of the Invention
The present invention relates to pumps, and more particularly to pumps adapted to pump high-viscosity liquids and slurries.
2. Description of the Prior Art
Semi-solid colloidal dispersions of water-bearing blasting agents, e.g., water gels or slurry explosives or emulsion-type blasting agents, currently are available in the form of small-diameter cartridges. The cartridge, often referred to as a "chub" cartridge, is a tube of plastic film, filled with blasting agent, and gathered at both ends and closed, e.g., by means of metal closure bands around the gathered portions.
A machine which is capable of producing chub packages on a continuous basis is described in U.S. Pat. No. 2,831,302. The production of compartmented chub packages, such as those which are used in resin-anchored rock bolt mine-roof-support systems, is described in U.S. Pat. No. 3,795,801. These packaging machines, known as "form/fill" machines, continuously form a web of film into a single- or double-compartment tube and simultaneously fill the tube with product. They also constrict the tube at spaced intervals and apply the closure bands to each constricted area.
The capability of the pump used to deliver the product into the tube critically affects the packaging results. It goes without saying that the pump must provide accurate metering. In this instance, it must also be well-suited to the handling of high-viscosity (e.g., 10,000 to 5,000,000 cp), often abrasive, slurries. Beyond these requirements, however, is the important consideration of uniformity of flow rate. Because the tube-forming, filling, and closing operations have to be performed in proper synchrony, the flow rate of the product being pumped must be constant and equal to the rate at which the tube is formed and moves through the packaging machine. This produces a firm, usable package. If the pumping rate drops periodically, the resulting packages may be underfilled and limp. On the other hand, if the pumping rate is excessive, the packages may break. Deviations in flow rate as small as 1-2% can create difficulties in package use.
A constant flow rate of pumped product is important in pumping many types of products in addition to water-bearing explosives and roof bolt anchoring compositions. These include food products, concrete, fraccing fluids for oil and gas wells, coal/water slurries, nuclear waste slurries, asphalt, paint, and filled epoxy resins.
Many pumps are available which have a good metering capability. These include gear pumps, piston pumps, and screw pumps. However, pumps such as these generally do not handle slurries well, particularly when they are high in viscosity and abrasive. Moreover, the known diaphragm pumps that will handle slurries all suffer from one drawback: they do not provide a fully constant flow rate.
For example, the pump described in U.S. Pat. No. 2,419,993 includes two chambers, each having a flexible diaphragm separating it into two compartments containing the delivery fluid (fluid to be pumped) and the driving fluid. However, because of the simultaneous switching of the valves in the driving fluid lines, the compressibility of the fluids, the expansion of the housings, and the movement of the check valves used, the flow of delivery fluid at changeover from one diaphragm to the other is pulsating. Thus, two pulses in flow occur during each cycle. Likewise, in the diaphragm pump described in U.S. Pat. No. 3,646,000, which employs four diaphragms, a pressure pulse is created when each pair of diaphragms reverses direction. This, coupled with the action of check valves, causes a pulsating flow.
The twin-diaphragm pump shown in U.S. Pat. No. 2,667,129 also is incapable of providing a constant flow rate owing to its check valves and the mechanical linkage of the diaphragms. The pumping ceases momentarily when the direction of motion is reversed. The diaphragm-type mud pump of U.S. Pat. No. 2,703,055 also has no constant-flow capability because of the check valves, the compressibilty of the fluids, the expansion of the housings, and the simultaneous switching from one housing to the other. The change in internal volume in the switching of the valves in the pump described in U.S. Pat. No. 3,320,901 prevents a constant flow rate from being achieved on switching from one cylinder to another.
Other patents on slurry pumps that also exhibit one or more of the above deficiencies include U.S. Pat. Nos. 3,637,328, 3,951,572, and 4,321,016.
The pulsating flow problem encountered in the above-described pumps could be reduced by using three or more pumping chambers, but this would entail great complexity and expense. Moreover, the valves used in these pumps are usually stated to be check valves, which require reverse fluid flow to close and which extract energy from the fluid, thus changing the flow rate momentarily. Furthermore, the flow rate drops during the changeover from one chamber to another due to the compressibility of the fluid and the expansion of the diaphragm housing. This drop in flow rate can be substantial, particularly if the slurry being pumped contains entrained air (as can be the case with slurry explosives) or if the pressures are very high.